Headlong by Kathe Koja

Lily has attended posh Vaughn school since pre-kindergarten. In her sophomore year, Lily's rebellion against being a perfect student and perfect daughter coincides with meeting Hazel, a new student at the school. The two girls plunge headlong into friendship as they test the limits of their emerging selves.

Lily's first person narration is occasionally interspersed with the perspective of various adults from her world, effecting a more balanced view for the reader. The boarding school setting with its cliques and pranking brings other teen novels to mind, like The Disreputable History of Frankie Landau-Banks and Looking for Alaska. Recommended.